Bipolarons in a Bose-Einstein Condensate

TL;DR

This paper demonstrates that two polarons in a Bose-Einstein condensate can form bound bipolarons through nonlocal interactions, with theoretical predictions supported by Quantum Monte Carlo simulations, and suggests experimental observability.

Contribution

The study develops a field-theoretic framework for bipolaron formation in BECs, extending beyond weak-coupling approximations and providing a method to observe these states experimentally.

Findings

Bipolarons form via nonlocal density oscillation-mediated interactions.

Quantum Monte Carlo results agree with the theoretical predictions.

Bipolaron formation requires strong impurity interactions, beyond weak-coupling models.

Abstract

Mobile impurities in a Bose-Einstein condensate form quasiparticles called polarons. Here, we show that two such polarons can bind to form a bound bipolaron state. Its emergence is caused by an induced nonlocal interaction mediated by density oscillations in the condensate, and we derive using field theory an effective Schr\"odinger equation describing this for arbitrarily strong impurity-boson interaction. We furthermore compare with Quantum Monte Carlo simulations finding remarkable agreement, which underlines the predictive power of the developed theory. It is found that bipolaron formation typically requires strong impurity interactions beyond the validity of more commonly used weak-coupling approaches that lead to local Yukawa-type interactions. We predict that the bipolarons are observable in present experiments and describe a procedure to probe their properties.